Despite recent advances in antimicrobial therapy, the mortality associated with bactermia, including postsurgical sepsis, remains between 25% and 50%. Recent studies have reported that both systemic gram-negative sepsis and recurrent intra-abdominal infections can originate from the patient's normal gastrointestinal flora. It is important to define the mechanism(s) whereby bacteria are normally contained within and yet some time escape (translocate) the intestinal tract and cause systemic disease. We know that relatively few of the 500 enteric species translocate with any frequency, but that these bacteria are the most frequent etiologic agents of bacteremia and postsurgical sepsis (E. coli, enterococci, staphylococci, Candida). In several animal models, translocating bacteria can be most reliably detected in mesenteric lymph nodes, and translocation is increased either after a shift in the intestinal flora or after immunosuppressive insults including trauma and burns. We have defined models whereby intestinal bacteria translocate either into mesenteric lymph nodes or into experimental intra-abdominal absesses initiated in normal rats. Inert particles such as ferritin, starch, and latex beads also translocate. We have two aims: (1) to determine the route(s) by which bacteria or inert particles leave the enteric lumen and pass into mesenteric lymph nodes or peritoneal inflammatory exudates; and (2) to determine the means by which the bacteria are contained. Initial experiments will be performed with inert particles because such experiments are technically easier. We can then focus detailed mechanistic studies on experiments with live bacteria. Our method includes feeding fluorescein labeled inert particles and bacteria (live and dead) and tracking the route of particle migration using: (a) a FLUOROSCAN, an instrument that grossly measure the presence of low concentrations of fluorescent particles in tissue and allows us to grossly determine the most relevant foci of intestinal tissue, (b) a flow cytometer, an instrument that sorts different cell types and measures the number of fluorescent particles within a given cell type; (c) traditional techniques of quantitation of viable bacteria in normally sterile sites; and (d) light and electron microscopy of translocating particles. Our working hypothesis is that intestinal bacteria are normally transported out of the intestinal lumen within phagocytic cells which fail to kill them only when local or systemic factors impair cellular immunity. Failure to kill the entrapped microbes permit them to spread locally to inflammatory foci or to the systemic circulation.